Method of making polyvinylidene fluoride membrane

ABSTRACT

A method of making a porous polymeric material by heating a mixture comprising polyvinylidene fluoride and a solvent system initially comprising a first component that is a latent solvent for polyvinylidene fluoride and a second component that is a non-solvent for polyvinylidene fluoride wherein, at elevated temperature, polyvinylidene fluoride dissolves in the solvent system to provide an optically clear solution. The solution is then rapidly cooled so that non-equilibrium liquid-liquid phase separation takes place to form a continuous polymer rich phase and a continuous polymer lean phase with the two phases being intermingled in the form of bicontinuous matrix of large interfacial area, and cooling is continued until the polymer rich phase solidifies. The polymer lean phase is removed from the solid polymeric material. A porous material so prepared is characterised by a lacey or filamentous structure consisting of a plurality of polymer strands (1) connected together at a number of locations (2) spaced apart along each strand.

This application is a Continuation-in-Part of applicant's applications,U.S. Ser. No. 07/536,650, filed Jul. 9, 1990 and having an InternationalFiling Date of Nov. 10, 1989, now U.S. Pat. No. 5,318,417, issued Jun.7, 1994, and U.S. Ser. No. 07/941,376, filed Sep. 4, 1992, now U.S. Pat.No. 5,277,851, issued Jan. 11, 1994, which is in turn a Continuation ofU.S. Ser. No. 07,536,649, filed Jul. 9, 1990 and having an InternationalFiling Date of Nov. 10, 1989, now abandoned, benefit of which are herebyclaimed under the provisions of 35 U.S.C. 120.

FIELD OF INVENTION

This invention relates to porous polymeric membranes and moreparticularly to such membranes that are prepared from polyvinylidenefluoride.

Polyvinylidene fluoride is a well known polymer of general formula (C₂H₂ F₂)_(n). It has the advantages of strength and oxidation resistance.

BACKGROUND ART

Polymeric membranes may be prepared by the phase inversion techniquewhich commences with the formation of a molecularly homogeneous, singlephase solution of a polymer in a solvent. The solution is then allowedto undergo transition into a heterogeneous, metastable mixture of twointerspersed liquid phases one of which subsequently forms a gel. Phaseinversion can be achieved by solvent evaporation, non-solventprecipitation and thermal precipitation.

The quickest procedure for forming a microporous system is thermalprecipitation of a two component mixture, in which the solution isformed by dissolving a thermoplastic polymer in a solvent which willdissolve the polymer at an elevated temperature but will not do so atlower temperatures. Such a solvent is often called a latent solvent forthe polymer. The solution is cooled and, at a specific temperature whichdepends upon the rate of cooling, phase separation occurs and the liquidpolymer separates from the solvent.

All practical thermal precipitation methods follow this general processwhich is reviewed by Smolders et al in Kolloid Z.u.Z Polymer, 43, 14-20(1971). The article distinguishes between spinodal and binodaldecomposition of a polymer solution.

The equilibrium condition for liquid-liquid phase separation is definedby the binodal curve for the polymer/solvent system. Forbtnoda/decomposition to occur, the solution of a polymer in a solvent iscooled at an extremely slow rate until a temperature is reached belowwhich phase separation occurs and the liquid polymer separates from thesolvent.

It is more usual for the phases not to be pure solvent and pure polymersince there is still some solubility of the polymer in the solvent andsolvent in the polymer, there is a polymer rich phase and a polymer poorphase. For the purposes of this discussion, the polymer rich phase willbe referred to as the polymer phase and the polymer poor phase will bereferred to as the solvent phase.

When the rate of cooling is comparatively fast, the temperature at whichthe phase separation occurs is generally lower than in the binodal caseand the resulting phase separation is called spinodal decomposition.

According to the process disclosed in U.S. Specification No. 4,247,498,the relative polymer and solvent concentrations are such that phaseseparation results in fine droplets of solvent forming in a continuouspolymer phase. These fine droplets form the cells of the membrane. Ascooling continues, the polymer freezes around the solvent droplets.

As the temperature is lowered, these solubilities decrease and more andmore solvent droplets appear in the polymer matrix. Syneresis of thesolvent from the polymer results in shrinkage and cracking, thus forminginterconnections or pores between the cells. Further cooling sets thepolymer. Finally, the solvent is removed from the structure.

Known thermal precipitation methods of porous membrane formation dependon the liquid polymer separating from the solvent followed by cooling sothat the solidified polymer can then be separated from the solvent.Whether the solvent is liquid or solid when it is removed from thepolymer depends on the temperature at which the operation is conductedand the melting temperature of the solvent.

True solutions require that there be a solvent and a solute. The solventconstitutes a continuous phase and the solute is uniformly distributedin the solvent with no solute-solute interation. Such a situation isalmost unknown with the polymer solutions. Long polymer chains tend toform temporary interactions or bonds with other polymer chains withwhich they come into contact. Polymer solutions are thus rarely truesolutions but lie somewhere between true solutions and mixtures.

In many cases it is also difficult to state which is the solvent andwhich is the solute. In the art, it is accepted practice to call amixture of polymer and solvent a solution if it is optically clearwithout obvious inclusions of either phase in the other. By opticallyclear, the skilled artisan will understand that polymer solutions canhave some well known light scattering due to the existence of largepolymer chains. Phase separation is then taken to be that point, knownas the cloud point, where there is an optically detectable separation.It is also accepted practice to refer to the polymer as the solute andthe material with which it is mixed to form the homogeneous solution asthe solvent.

There are several characteristics in the morphology of a membrane thatcan describe what is observed when phase inversion membranes arescrutinised under an electron microscope. The morphologicalcharacteristics may be described with the terms symmetry, homogeneityand isotropy.

Symmetry means that one half of the structure is the mirror image of theother half. The device about which a membrane is symmetrical is a planeor surface half way between the two faces of the membrane. In membranescience, the term is often incorrectly used to mean homogeneous.Homogeneous means simply that the membrane has a uniform structure. Inchemistry, the term "homogeneous", when ascribed to a substance, meansthat it has uniform structure or composition.

Isotropic means that the membrane has equal properties in alldirections. The word isotropic comes from biology where it is means atendency for equal growth in all directions.

The opposite of these terms are often used, namely--asymmetric,non-homogeneous and anisotropic. Anisotropic is often incorrectlyunderstood to mean asymmetric or non-homogeneous. Anisotropic morecurrently describes how a morphology develops rather than the nature ofthe morphology.

In membrane science, the meaning of the above words has been refined bytechnological development. Prior to about 1960, phase inversionmembranes were isotropic or only slightly anistropic. About that time,membranes with more inhomogeneity were developed.

Taking a vector from one face of a membrane to another, there are twotypes of inhomogeneity of importance known in membrane science asskinning and anisotropy.

Skinning is used as a synonym for asymmetry, and refers to a membranehaving a relatively thin dense layer at one surface of the membrane witha relatively thick porous substructure throughout the remainder of themembrane. The first skinned membrane made by phase inversion isdescribed in U.S. Specification No. 3,133,132 which discloses thesolvent intrusion method of phase inversion.

In addition to the terms used to describe how one region of porousmembrane is related to another, there are more specialised terms used indescribing the shapes of the pores themselves.

Membrane scientists use the word structure when referring to the shapesof pores, cells, alveoli and other void shapes found within themembrane. The structure can be described as granular, spongy,reticulate, or lacey. The voids can be described as cells, or cells withinterconnecting pores, and larger cavities can be described asmacrovoids.

When viewed under an electron microscope, granular structures arecharacterised by polymer balls roughly spherical in shape which appearto be fused together as if sintered. Granular structures are notgenerally desirable in microporous membranes because the porosity andmechanical strength are both lower than other types of structure.

A spongy structure is characterised by roughly spherical cells connectedby roughly cylindrical conduits or pores. Such a structure is disclosedin U.S. Specification No. 4,519,909.

A reticulate structure is characterised by a netlike appearance.

On the other hand, the polymertc material which forms the substance of alacey structure can be described as multiply connected strands ofpolymer, with each connection point having only slightly largerdimensions that the cross-section of the strands. The strands have alength substantially larger than the largest cross-sectional dimension,and the shape of the cross-section of the strands varies from strand tostrand as well as along the strand. The shape of the cross-section ofthe strands can be described as round or ensiform, circular or oval. Thestrands may have grooves or furrows, or even appear to be like amultiplicity of coalsaced filaments.

All of the above structures are bicontinuous in the solid state in thatevery part of the polymer is connected to every other part of thepolymer, and every cavity is connected to every other cavity in anintermingled porous network of polymer and cavity.

As well as the above structures, interposed upon granular, spongy andlacey structures there can be cavities of substantially largerdimensions than those described earlier, and these cavities are referredto as macrovoids. Macrovoids which are elongated in shape are calledfinger voids, and macrovoids which are rounder in shape are calledalveoli.

Macrovoids are, by definition, completely surrounded by the microporousstructure of the membrane.

Several membranes made of polyvinylidene fluoride have been cited in theliterature. Most are sheet membranes which are made by the commonprocess of non-solvent (or poor solvent) intrusion to cause Gelation orphase inversion.

For example, U.S. Pat. No. 3,642,668 discloses dimethyl sulfoxide (DMSO)or dimethyl acetamide (DMAc) as the solvent for polyvinylidene fluoridewhen casting a membrane onto a support structure, immediately followedby immersion in a non-solvent bath, typically methanol.

Japanese Patent No. 51-8268 uses cyclohexanone as a solvent forpolyvinylidene fluoride. The solution is heated and then cooled duringwhich time the solution passes through a region of maximum viscosity.The membrane is cast when the viscosity of the solution is decreasing.

European Patent No. 223,709 discloses a mixture of acetone and dimethylformamide (DMF) as a preferred solvent although all the usual standardor active solvents such as ketones, ethers such as tetrahydrofuran and1,4 dioxane, and amides such as DMF, DMAC and DMSO are described. Themembrane is formed by coating the polymer solution onto a substratewhich is immediately immersed in a poor solvent.

In the process disclosed in U.S. Pat. No. 4,203,847 flat sheet membranesare formed by casting a nearly saturated solution in hot acetone onto amoving belt which then passes into a forming bath containing a mixtureof solvent and non-solvent. This produces a thin skinned membrane. U.S.Pat. No. No. 4,203,848 describes the belt and machine used in thisprocess.

U.S. Pat. No. 3,518,332 discloses a flat sheet membrane made by pressingand sintering a mixture of polyvinylidene fluoride particles withparticles of a metallic salt and paraffin wax.

U.S. Pat. No. 4,810,384 describes a process wherein polyvinylidenefluoride and a hydrophilic polymer compatible therewith are dissolved ina mixture of lithium chloride, water and dimethylformamide, then castonto a web and coagulated by passing the film through a water bath. Ahydrophilic membrane that is a blend of the two polymers is produced.

U.S. Pat. No. 4,399,035 discloses a polyvinylidene fluoride membraneprepared by casting a dope comprising polyvinylidene fluoride, an activesolvent such as DMAc, N-methylpyrrolidone or tetramethylurea and a minoramount of a surfactant or mixture of surfactants into a non-solventbath, typically water or an alcohol. Polyethylene glycol andpolypropylene glycol are used as surfactants and glycerin fatty acidesters are mentioned in the description as being suitable.

U.S. Pat. No. 4,666,607 describes a thermal gelation process. Itdiscloses the use of a quench tube in the form of a U-tube, or a tankwith the fibre moving as if in a U-tube, which can be used to producepolyvinylidene fluoride films or hollow fibres by extrusion of asolution comprising the polymer, solvent(s) and a non-solvent above thetemperature at which the solution will separate into two phases,advantageously through an air gap into a cooling liquid in the quenchtube or tank. In the case of hollow fibres, a lumen forming fluid (whichis not a solvent for the polymer) is employed.

Emphasis is placed on the avoidance of stress on the extruded, but stillliquid, fibre and the stretch factor (i.e. the ratio of the velocity ofthe formed, cooled fibre membrane to the velocity of the polymersolution emerging from the forming die) is typically in the region ofonly 1.33.

DISCLOSURE OF THE INVENTION

According to the invention, there is provided a method of making aporous polymeric material comprising the steps of:

(a) heating a mixture comprising polyvinylidene fluoride and a solventsystem initially comprising a first component that is a latent solventfor polyvinylidene fluoride and a second component that is a non-solventfor polyvinylidene fluoride wherein, at elevated temperature,polyvinylidene fluoride dissolves in the solvent system to provide anoptically clear solution,

(b) rapidly cooling the solution so that non-equilibrium liquid-liquidphase separation takes place to form a continuous polymer rich phase anda continuous polymer lean phase with the two phases being intermingledin the form of bicontinuous matrix of large interfacial area,

(c) continuing cooling until the polymer rich phase solidifies,

(d) removing the polymer lean phase from the solid polymeric material.

A latent solvent is a solvent which will dissolve the polymer at anelevated temperature but allow the polymer to precipitate at lowertemperatures.

Preferably, the latent solvent is a glycerol ester such as glyceroltriacetate (KODAFLEX TRIACETIN®--a Trademark), glycerol tributyrate,glycerol tripropionate or partially-esterified glycerol).

The preferred latent solvent is glycerol triacetate (GTA).

The mixture may additionally contain an antioxidant. Possibleantioxidants are selected from the group of hindered phenolantioxidants. Preferred antioxidants are ETHANOX 330®1,3,5-trimethy-2,4,6-tris-3,5-di-tert-butyl-4-hydroxybenzyl) benzene)and ULTRANOX TM® (Bis (2,4-di-tert-butyl phenyl) pentaerythritoldiphosphite) or mixtures thereof. Ethanox 330 is particularly preferred.

Typically, the mixture is heated for about 1 to about 20 hours under apressure substantially below atmospheric that is governed by the vapourpressure of the solvent system.

The non-solvent may be selected from high boiling point, somewhat polarand hydrogen bonding compounds such as higher alcohols, glycols andpolyols.

The non-solvent may be glycerol, diethylene glycol, dipropylene glycol,polyethylene glycol or polypropylene glycol.

In a preferred form of the invention, the polyvinylidene fluoride isdissolved in a mixture of glycerol triacetate and diethylene glycol(known by the trivial name digol).

In this invention, a combination of polymer and a solvent system wasemployed from which, on rapid cooling, a bicontinuous matrix of twoliquids was found to occur. With the correct solvent properties for aselected polymer, non-equilibrium liquid-liquid phase separation takesplace to form a bicontinuous matrix of polymer rich and polymer leanphases. This is in contrast to the mechanism of nucleation and growthwhich occurs in prior art thermal precipitation phase inversionmembranes. This is supported experimentally by differential scanningcalorimetry (DSC) testing, which showed that there is neither anendotherm nor exotherm during the liquid phase separation as would beexpected if nucleation/crystallization took place.

The present invention differs from most prior art in that it relies ongelling the polymer by lowering the temperature (i.e. thermal gelation),not on non-solvent intrusion. Consequently, the present invention cannotuse an active solvent (one which will dissolve the polymer at anytemperature) as was done largely in the prior art, but must use asolvent system that is or contains a latent solvent.

While not wishing to be bound by theory, it is believed that a chemicalreaction occurs between the components of the solvent system. It hasbeen shown, by both gas- and thin-layer chromatography, that a mixtureof up to nine reaction products may be formed and it may be that themixture of reaction products collectively, or some component orcomponents of the reaction mixture, constitute the latent solvent forpolyvinylidene fluoride.

According to a second aspect of the invention there is provided a porouspolyvinylidene fluoride material characterised by a lacey or filamentousstructure consisting of a plurality of polymer strands connectedtogether at a number of locations spaced apart along each strand.

Typically, each connection point has only slightly larger dimensionsthan the cross-section of the strands. The length of each strand is from5 to 50 times the diameter of the strand and the strands vary incross-sectional shape from circular to elliptical, in the latter casethe major axis of the ellipse may be up to five times the minor axis ofthe ellipse. The description "lacy or filamentous structure" may also bevisualteed as a three dimensional rounded lace filet derived from abicontinuous structure.

In prior art membranes, a spongy structure can be obtained from anysystem which has a miscibiltty gap, and open celled pore connections aredue to shrinkage and syneresis, whereas according to the presentinvention, the lacey structure of controlled morphology can be obtainedonly where there is a liquid-liquid bicontinuous phase separation.

In a preferred form of the invention, the membrane is a hollow fibremembrane which has a lacey structure in which there is some orientationof the strands in the axial direction of the fibre so that when alumenal gas backwash procedure is implemented to clean the fibres,certain dimensions of the interstices increase on average allowing anymaterial lodged in the interstices to be easily dislodged. Theinterstices are of a generally axially elongated shape and when the gasbackwash is applied, the interstices distort from the axially elongatedshape into a generally square shape to enlarge the minimum dimension ofthe interstices. The gas backwash will also stretch the fibre toincrease the minimum dimension of the interstices.

Advantageously, the gas backwash is applied by pulsing air for 1 to 5seconds at a pressure of about 600 kPa through the lumen of a hollowfibre membrane to cause explosive decompression through the walls of thefibre, thereby dislodging retained solids from the fibre. This gasbackwash may be preceded by a pressurised reverse flow of liquid.

In a preferred form of the invention, the porous polyvinylidene fluoridematerial is formed as a hollow fibre using a quadruple co-extrusion headhaving four concentric passageways. The axial passageway contains alumen forming fluid. The first outwardly concentric passageway containsa homogenous mixture of the polymer and solvent system to form themembrane, the next outwardly concentric passageway has a coating fluidand the outermost passageway has a cold quench fluid. Preferably, thelumen, coating and quenching fluids contain the solvent systemcomponents in selected proportions (the first component may be absent).The composition of the coating and lumen fluids predetermine the poresize and frequency of pores on the membrane surfaces.

Each fluid is transported to the extrusion head by means of individualmetering pumps. The four components are individually heated and aretransported along thermally insulated and heat traced pipes. Theextrusion head has a number of temperature zones. The lumen fluid,membrane forming solution (dope) and coating fluid are brought tosubstantially the same temperature in a closely monitored temperaturezone where the dope is shaped. The quench fluid is introduced at asubstantially lower temperature in a cooling zone where the dopeundergoes non-equilibrium liquid-liquid phase separation to form abicontinuous matrix of large interfacial area of two liquids in whichthe polymer rich phase is solidified before aggregated separation intodistinct phases of small interfacial area can take place.

Preferably, any air, gas or vapour (not being a gas or vapour thatserves as the lumen fluid), is excluded during extrusion and the fibreis stressed axially to stretch it by a factor ranging from 5 to 100,thereby elongating the surface pores.

It is to be noted that the fibre travels down the quench tube at asignificantly different linear speed from the quench fluid. The extrudedfibre travels at a speed three to four times faster than the averagespeed of the quench fluid. Such a speed difference calculated on theaverage speed also means that the fibre travels at a speed about doublethe maximum speed of the quench fluid. The average and maximum speed ofthe quench fluid above are taken as the speed with no fibre present.

The hollow fibre membrane leaves the extrusion head completely formedand there is no need for any further formation treatment except forremoving the solvent system from the membrane in a post-extrusionoperation that is common to membrane manufacturing process. In apreferred method, an appropriate solvent that does not dissolve thepolymer but is miscible with the dope solvents is used to remove thesolvent system for the polymer from the finished membrane. In aparticularly preferred method, water at 80°-100° C. is used.

The lumen forming fluid may be selected from a wide variety ofsubstances such as soybean oil and an inert gas such as nitrogen. Thesame substance may be used as the coating and quenching liquids. Wateror virtually any other liquid may be used as the quench liquid. Othersubstances which may be used as the lumen forming material, the coatingliquid and the quenching liquid include:

(a) dioctylphthalate and other phthalate esters of alcohols of sixcarbon atoms or more

(b) diethylene glycol

(c) dipropylene glycol

(d) diethylene glycol and glycerol triacetate

(e) dipropylene glycol and glycerol triacetate

(f) polyethylene glycol

(g) polypropylene glycol.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more readily understood and put intopractical effect, reference will now be made to the accompanyingdrawings in which:

FIG. 1a is a micrograph of the surface of the membrane produced inexample 1.

FIG. 1b is a micrograph of a cross section of the membrane of example 1.

FIG. 2a is a micrograph of the surface of the membrane produced inexample 2.

FIG. 2b is a micrograph of a cross section of the membrane produced inexample 2.

FIG. 3 is a schematic diagram of an extrusion die according to theinvention,

FIG. 4 is a cross-sectional view of an extrusion die assembly accordingto one embodiment of the invention,

FIG. 5 is an enlarged cross-sectional view of the upper or melt dieportion of the extrusion die assembly of FIG. 4, and,

FIG. 6 is an enlarged cross-sectional view of the lower or quench tubeportion of the extrusion die shown in FIG. 4,

FIG. 7 is an enlarged cross-sectional view of the discharge nozzle ofthe melt die portion of the extrusion die assembly shown in FIG. 4.

Referring to FIGS. 1a and 2a, the resemblance of the surface of themembrane to lace material can be seen. The polymer strands are joinedtogether at intervals by bridges of polymeric material just as in alacey handkerchief or the like.

As can be seen, the strand does not broaden substantially at theconnection point between the strand and the bridge. Orientation of thestrands in the axial direction of the fibre is clearly evident in FIG.1a where all of the strands run almost parallel in the axial direction.

An alveolus (singular of alveoli) is present in FIG. 1b.

The extrusion die shown in schematic form in FIG. 3 has, at its upperend, three concentric passageways 11, 12 and 13. The axial passageway 11carries a lumen fluid 14, the inner annular passageway 12 carries anoptically clear solution (or dope) 15 of polyvinylidene fluoride andsolvent system and the outer annular passageway 13 carries a hot coatingfluid 16. The thick lines in FIG. 3 represent walls and the thin linesrepresent interfaces between the various fluids.

The upper portion 17 of the extrusion die is a closely monitoredtemperature zone. Within the hot zone 17, the coating material-remainsas a coating on the membrane 21 being formed and modifies the surface ofthe membrane 21 to provide a porous surface on the membrane.

Below the hot zone 14 there is a cooling zone 18 which includes anannular quench fluid passageway 19. The quench fluid is pumped throughthe quench passageway 19 at a fixed rate and the coolant or quench fluidis not open to the atmosphere. The inner wall of quench passageway 19has a series of openings 20 through which the quench fluid passes.Beyond the extrusion die there is a collector for receiving the extrudedmembrane 21.

An extrusion die assembly 30 according to one embodiment of theinvention is shown in FIGS. 4 to 7 and consists of an upper or melt dieportion 31 and a lower or quench tube die portion 32 coupled together bya union 33.

The melt die portion 31 which is shown on an enlarged scale in FIG. 5,has a body 34 having an inlet 35 for receiving membrane forming dope andan inlet 36 for receiving coating fluid. The body has a central bore 37and at its upper end there is a closure plate 38 having an axialpassageway 39 for receiving a lumen forming fluid. The plate 38 issecured to the body 34 by bolts 40 and a seal is provided by "O" ring41.

Within the central bore 37 of the body 34 there is a nozzle member 42which depends from the plate 38. The axial passageway 39 is reduced indiameter at its lower end where it passes through the tapered end 43 ofthe nozzle member 42. The nozzle member 42 is sealed in the body 34 by"O" ring 44. The passageway 39 corresponds to passageway 11 of FIG. 3.

The dope inlet 35 leads to a dope delivery passageway 45 incommunication with an annular chamber 46 formed in the outer surface ofnozzle 42. Dope is discharged from the chamber 46 into passageway 47which exits into a tapered annular fibre forming tube 48 defined betweenthe outer face of the nozzle 42 and a recess 49 formed in die plate 50.As can be seen in FIGS. 5 and 7 the fibre forming tube 48 has an upperconical portion 48a and a lower conical portion 48b. The upper portion48a is inclined at a larger angle to the vertical than the lower portion48b. In this instance, the angle of inclination of the upper portion isfrom 30° to 60° from the axis and that of the lower portion is from 1°to 10° from the axis. In the preferred embodiment, the angle from theaxis on the upper portion of nozzle 42 is 44° and on the upper portionof the die plate 50 is 50° and on the lower portion of nozzle 42 is 3°and on the lower portion of ringplate 50 is 5°. The tapered tube 48provides a neck-down ratio (that is the ratio of the diameter of themolten dope at the bottom of the tube 48 to diameter of the finishedfibre) of 3.8 to 1. The neck down ratio may be in the range of 1.4:1 to10:1.

The coating fluid inlet 36 leads to a coating fluid delivery passageway51 in communication with an annular chamber 52 formed by a recess in thebottom of the body 34 and the die plate 50. Coating fluid is dischargedfrom chamber 52 into passageways 53 formed in the die plate 50 whichexit into an annular chamber 54 formed between the bottom of the dieplate 50 and ring plate 51.

The ring plate 51 is secured to the body 34 by bolt 55. "O" ring 56provides a seal between the ring plate 51, die plate 50 and body 34 and"O" ring 57 provides a seal between die plates 50 and body 34. A centralbore 58 of the stem portion 59 of the ring plate 51 receives the fibrewhich is retained in hollow form by the lumen fluid and which is coatedwith the coating fluid.

The quench tube portion 32 which is shown on an enlarged scale in FIG. 6has a body portion 60 and a connector plate 61 secured thereto by bolt62. "O" ring 63 provides a seal between the body 60 and plate 61. Thebody 60 has a quench fluid inlet 64 which leads to a quench fluidchamber 65 formed by a recess 66 is formed in the body 60.

Within the recess 66 there is a quench oil diffusor 67 having an axialbore 68. Passageways 69 connect the chamber 65 to the bore 68.

"O" ring 70 seals the diffusor 67 with respect to the body 60 and "O"ring 71 seals the diffusion 67 with respect to the connector plate 61.The bore 68 of the diffusor 67 is in communication with the bore 72 ofbody 60 which in turn is in communication with the bore 73 of dischargetube 74.

FIG. 7 is an enlarged view of the discharge nozzle 42 which, in thisinstance, is modified to be in the nature of a needle 80 having aplurality of protrusions 81 which act to self centre the needle 80within the chamber 48.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will now be described with reference to the production ofporous hollow fibre membranes.

Example 1

A hollow fibre polyvinylidene fluoride membrane was prepared using theextrusion apparatus illustrated in FIGS. 4 to 7. A mixture of 30.0%KYNAR® 461® (a trademark for polyvinylidene fluoride), 30% glyceroltriacetate, 39.9% Digol (the trivial name for diethylene glycol) and0.1% ETHANOX 330® (Ethanox 330 is a trade mark for1,3,5-trimethyl-2,4,6-tris-3,5-di-tert-butyl-4-hydroxybenzyl) benzene)as an antioxidant was prepared and then mixed together whilst heatingunder partial vacuum to a temperature of 220° C. to form a dope. Thedope was held at this temperature in a holding tank (not shown) whilebeing progressively introduced to the extruder through inlet 35. Theflow rate of the dope was 20 cc/min. and the extrusion temperature was215° C.

A lumen forming fluid (digol) which enters the extruder through inlet 39and ultimately passes through the tapered end 43 of nozzle 42 wasintroduced. As dope is discharged from fibre-forming tube 48 intocentral bore 58, the role of the lumen forming fluid discharged fromnozzle 42 is to maintain the lumen in the hollow fibre being formed. Theflow rate of the/umen-forming fluid was 6.0 cc/min.

As the hollow fibre was extruded, a coating fluid comprising a mixtureof 10% glycerol triacetate and 90% digol was discharged from passageway53 to coat the formed hollow fibre as it entered the central bore. Theflow rate of the coating fluid was 15 cc/min.

Both lumen-forming and coating fluids were at essentially the sametemperature as the dope.

The formed hollow fibre passes through the central bore of the extruderto the quench region shown in FIG. 6 where digol was used as the quenchfluid. The temperature of the digol was about 67° C. and it wasintroduced at a flow rate of 800 cc/min.

The hollow fibre was discharged from the extruder at a haul-off rate of60 m/min. As the velocity of the extruded dope is 5.8 m/min there was adrawdown factor of 10.3 and substantial stretching of the fibreoccurred.

The finished fibre had a pore size of 0.3 micron and a waterpermeability of 141 ml/min/m at a pressure of 100 kPa. The membrane hada lacey structure and orientation of the strands was evident. Thesefeatures are clearly seen in FIG. 1a and FIG. 1b.

Example 2

35% of KYNAR 461® and 0.1% of ETHANOX 330® were dissolved in 30% GTA and34.9% Digol at 225° C. This was extruded at 215° C. as the second streamin the extruder of Example 1. The first and fourth streams were Digolbut the third stream was 50/50 GTA/Digol. Fibre was produced at 60meter/minute with a mean pore size of 0.21 microns.

Example 3

A dope comprising 11.75% KYNAR 461®, 11.75% SOLEF 1015® (a trademark forpolyvinylidene fluoride), 30% glycerol triacetate, 46.4% digol and 0.1%ETHANOX 300® was extruded at 220° C. as the second stream in theextruder of Example 1. The flow rate was 23 cc/min.

The lumen and quench streams were both digol and the flow rate of thesestreams was 5.5 cc/min and 300 cc/min respectively. The coating liquidwas a 50/50 mixture of glycerol triacetate and digol. The flow rate ofthe coating stream was 8 cc/min. The quench liquid was at a temperatureof 30° C.

The fibre was hauled off the extruder at 60 m/min. As the velocity ofthe extruded dope was 6.7 m/min there was a drawdown factor of 9 andsubstantial stretching of the fibre occurred.

The finished fibre has a pore size of 0.29 micron and a waterpermeability of 170 ml/min/m at a pressure of 100 kPa. The fibre lumendiameter was 0.35 mm and its outer diameter was 0.65 mm.

Example 4

Using the same, respective compositions of dope and other fluids as inExample 3, but reducing the dope flow and lumen flow rates by one-third,i.e. to 15.33 and 3.67 mis/min respectively, and correspondinglyreducing the fibre speed to 40 meters/minute to maintain the samedrawdown factor and fibre dimensions, the resulting fibre had a meanpore size of 0.24 micron, and a water permeability of 135mls/min/meter/100 kPa. The change in membrane properties compared toExample 3 can be attributed to the higher coating fluid and quench fluidflow rates relative to the flow rate of the extruded dope.

Example 5

The same respective compositions of dope, lumen and quench fluids wereused as in Example 3. However, the coating fluid was changed to a 60/40Digol/GTA mix. All other operating conditions were essentially the sameas for Example 3. The resulting fibre had a mean pore size of 0.37micrometers and a water permeability rate of 262 mls/min/metre/100 kPa.The change in membrane properties compared to Example 3 can beattributed to the different composition of the coating fluid mixture.

Example 6

Example 5 was repeated with the only change being a reduction in thetemperatures of the dope, lumen and coatings fluids to 210° C. Theresulting fibre had a mean pore size of 0.30 micrometers and a waterpermeability rate of 183 mls/min/meter/100 kPa.

Example 7

A solution (dope) was made at 220° C. comprising 12.5% KYNAR 461®, 12.5%Solef 1015, 30% GTA, 44.9% Digol, and 0.1% antioxidant (ETHANOX 330®).This was extruded at 220° C. as the second stream in the apparatus usedin the previous examples. The first (lumen) and fourth (quench) streamswere Digol, while the third (coating).stream comprised a mixture of57/43 Digol/GTA. The/umen and coating streams were at essentially thesame temperature as the dope stream, whereas the quench temperature was28° C. The flow rates of the dope, lumen, coating, and quench streamswere respectively 23, 7, 10 and 500 mls/min.

The fibre was produced at 60 meters/minute and had a mean pore size of0.28 micrometers and a water permeability rate of 160 mls/min/meter/100kPa.

Example 8

A solution (dope) was made at 210° C. comprising KYNAR 461®, 30.0% GTA,39.9% Digol, and 0.1% antioxidant. This was extruded at 210° C. in theapparatus used in the previous examples. Digol was used for the lumenand quench streams and a 57/43 Digol/GTA mix used for the coatingstream. The lumen, dope and coating streams were essentially at the sametemperature, whereas the quench stream temperature was 60° C. The flowrates of the dope, lumen, coating and quench streams were respectively20, 6 15, and 700 m/s/min.

The fibre was produced at 60 meters/min with an extruded dope velocityof 5.8 meters/min, representing a drawdown factor of 10.3, and had apore size of 0.51 micrometers and a water permeability rate of 306mls/min/meter/100 kPa. The larger pore size is attributed largely to thehigher quench temperature than used in the previous examples.

The conditions under which the membrane is extruded in examples 1 to 8are summarised in Table 1. Additional examples 9, 10 and 11 which werecarried out following the procedure of example 1 are also summarised inTable 1.

Various modifications may be made in details of process steps andcomposition selection without departing from the scope or ambit of theinvention. For instance, although the specification primarily addressesthe use of PVdF homopolymers in a process for producing hollow fibremembranes, it should be apparent to the skilled addressee that PVdFcopolymers or mixtures with suitable polymers may be used and that theprocess may be adapted for the formation of flat sheet membranes.

                                      TABLE 1                                     __________________________________________________________________________    DOPE COMPOSITION                                                              PVdF                                                                                    SOL-  SOLVENT                   TEMPERATURES                                  EF    SYSTEM   COMPOSITION OF   DOPE                                Example                                                                            KYNAR                                                                              1015/1001                                                                           GTA DIGOL                                                                              OTHER STREAMS    EXTN                                                                              QUENCH                          Number                                                                             461 %                                                                              %     %   %    LUMEN                                                                              COATING                                                                             QUENCH                                                                              °C.                                                                        °C.                      __________________________________________________________________________    1    30.0 --    30  39.9 DIGOL                                                                              90 DIGOL                                                                            DIGOL 215 67                                                            10 GTA                                          2    35.0 --    30  34.9 DIGOL                                                                              50 DIGOL                                                                            DIGOL 215 47                                                            50 GTA                                          3    11.75                                                                              11.75 30  46.4 DIGOL                                                                              50 DIGOL                                                                            DIGOL 220 30                                                            50 GTA                                          4    11.75                                                                              11.75 30  46.4 DIGOL                                                                              50 DIGOL                                                                            DIGOL 220 30                                                            50 GTA                                          5    11.75                                                                              11.75 30  46.4 DIGOL                                                                              60 DIGOL                                                                            DIGOL 220 30                                                            40 GTA                                          6    11.75                                                                              11.75 30  46.4 DIGOL                                                                              60 DIGOL                                                                            DIGOL 210 30                                                            40 GTA                                          7    12.5 12.5  30  44.9 DIGOL                                                                              57 DIGOL                                                                            DIGOL 220 28                                                            43 GTA                                          8    30.0 --    30  39.9 DIGOL                                                                              57 DIGOL                                                                            DIGOL 210 60                                                            43 GTA                                          9    11.25                                                                              11.25 30  47.4 DIGOL                                                                              57 DIGOL                                                                            DIGOL 220 30                                                            43 GTA                                          10   11.25                                                                              11.25 30  47.4 DIGOL                                                                              50 DIGOL                                                                            DIGOL 220 29                                                            50 GTA                                          11   11.25                                                                              11.25 30  47.4 DIGOL                                                                              70 DIGOL                                                                            DIGOL 220 30                                                            30 GTA                                          __________________________________________________________________________                                  HAUL           ANTIOX-                                                        OFF  AVGE      IDANT                            Example  FLOW RATES CC/MIN    RATE PORE                                                                              WATER*                                                                              ETHANOX                          Number   DOPE                                                                              LUMEN                                                                              COATING                                                                             QUENCH                                                                              M/MIN                                                                              SIZE                                                                              PERM. 330%                             __________________________________________________________________________    1        20  6    15    800   60   0.3 141   0.1                              2        15  6    11    800   60   0.21                                                                               76   0.1                              3        23  5.5  8     300   60   0.29                                                                              170   0.1                              4        15.33                                                                             3.66 8     300   40   0.24                                                                              135   0.1                              5        23  5.5  8     300   60   0.37                                                                              262   0.1                              6        23  5.5  8     300   60   0.30                                                                              183   0.1                              7        23  7    10    500   60   0.28                                                                              160   0.1                              8        20  6    15    700   60   0.51                                                                              306   0.1                              9        23  5.5  8     300   60   0.38                                                                              198   0.1                              10       23  5.5  15    500   60   0.25                                                                               82   0.1                              11       23  5.5  15    500   60   0.29                                                                              141   0.1                              __________________________________________________________________________     *WATER PERMEABILITY UNITS: mls/min/meter @ 100 KPa                       

We claim:
 1. A method of making a porous polymeric hollow fiber membranecomprising the steps of: (a) heating a mixture comprising polyvinylidenefluoride (PVdF) and a solvent system, initially comprising a firstcomponent that is a latent solvent for PVdF and a second component thatis a non-solvent for PVdF, wherein at elevated temperature PVdFdissolves in the solvent system to provide an optically clearsolution,(b) introducing said solution into an extrusion head adapted toform the solution into a hollow fiber membrane form extruded co-axiallywith a lumen-forming fluid, a coating liquid that is introduced aroundthe outer surface of the hollow fiber and a cooling liquid that isintroduced around the coating liquid, (c) rapidly cooling the solutionso that non-equilibrium liquid-liquid phase separation takes place toform a continuous polymer rich phase and a continuous polymer lean phasewith the two phases being intermingled to form a bicontinuous matrix oflarge interfacial area; (d) continuing cooling until the polymer richphase solidifies to form a porous hollow fiber membrane, and (e)removing the solvent system from said porous hollow fiber membrane.
 2. Amethod according to claim 1 wherein the mixture additionally contains anantioxidant.
 3. A method according to claim 1 wherein the mixture isheated for a period of between about 1 and about 20 hours.
 4. A methodaccording to claim 1 wherein the coating liquid comprises a mixture ofthe latent solvent and the non-solvent that formed the solvent system,whereby the proportion of the latent solvent and non-solvent are chosento predetermine the pore size and frequency of the pores on the membranesurfaces.
 5. A method according to claim 1 or claim 4 wherein air, gasor vapour, other than gas or vapour serving as lumen fluid, is excludedduring extrusion.
 6. A method according to claim 1 wherein the fibre isstressed axially during the cooling step to stretch it by a factorranging from 5 to 100, thereby elongating the surface pores.
 7. A methodaccording to claim 1 wherein the first component of the solvent systemis a glycerol ester.
 8. A method according to claim 7 wherein said firstcomponent is selected from the group consisting of glycerol triacetate,glycerol tripropionate, glycerol tributyrate and partially-esterifiedglycerol.
 9. A method according to claim 1 wherein the second componentof the solvent system is a high boiling point, polar compound that iscapable of hydrogen bonding.
 10. A method according to claim 9 whereinsaid second component is a higher alcohol, glycol or polyol.
 11. Amethod according to claim 10 wherein said second component is selectedfrom the group consisting of glycerol, diethylene glycol, dipropyleneglycol, polyethylene glycol and polypropylene glycol.
 12. A methodaccording to claim 1 wherein the mixture comprises polyvinylidenefluoride, glycerol triacetate, diethylene glycol and an antioxidant.